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a) Dpto. Qufmica Inorgdnica. Facultad de Ciencias. Universidad de Valladolid. 47005-Valladolid (Spain). b) Dpto. Materia Condensada. Facultad de Ciencias. Universidad del Pals Vasco. Apdo. 644. 48080-Bilbao (Spain). c) Qufmica Orgdnica. Instituto de Ciencia de Materiales de Arag6n. Facultad de Ciencias. Universidad de Zaragoza-CSIC. 50009-Zaragoza (Spain). d) Dpto. Fisica Aplicada II. Facultad de Ciencias. Universidad del Pafs Vasco. Apdo. 644. 48080-Bilbao (Spain).

ABSTRACT Ferroelectric metallomesogens are a new type of ferroelectric liquid crystal which have unique properties arising from the presence of metal atoms. Here we report an overview of results based on the wide range of structural changes, either in the metallic centre or in the organic part, that these compounds offer. Ferroelectric salicylaldiminate derivatives, imine ortho-palladated dinuclear complexes and ortho-metallated imine-p3-diketone mononuclear compounds give rise to chiral smectic C ordered materials with macroscopic polarizations up to 200 nC.cm- 2 while their response times are in the range of Ms. Depending on the metal and the structure of the metallomesogens, additional properties such as paramagnetic behavior as well as non-linear optical reponses are described for these new materials. Thus, imine-13-diketone compounds of Pd(ll) and Pt(ll) are the first metallomesogens ever reported which show second harmonic generation. deff values in the range of 0.03 to 0.06 pm.V- 1 at 10 °C below the transition to the SmC* phase have been measured for these materials under phase matching conditions.

Introduction Until 1980, most of the reported liquid crystal materials were organic compounds. However, during the last decade, a new class of mesogens incorporating transition metals has burst into the liquid crystal field, yielding a great variety of mesophases. In addition such materials generate many different coordination geometries and molecular shapes. These compounds are called the metallomesogens [1]. By introducing chirality into these structures, chiral nematic (cholesteric) [2] and smectic [3-7] phases have been obtained. In the case of the smectic C phase, this allows the formation of ferroelectric fluid materials for which the 123

Mat. Res. Soc. Symp. Proc. Vol. 392 01995 Materials Research Society

presence of the metal atoms may also render special electrical, optical and/or magnetical properties. This communication intends to provide an overview of the main results on ferroelectric metallomesogens obtained by our group so far. The first ferroelectric metallomesogen Based on our studies on achiral metallomesogenic structures, the first metal-containing ferroelectric liquid crystals was synthesized at the end of the eighties. In 1989 we reported an organo-transition metal compound displaying ferroelectric behavior [3]. It was an ortho -palladated dimer derived from an azine with a chiral carboxilato intermetallic bridge (Figure 1).

Along with the presence of metal atoms in the molecule, two novel aspects may be noted for this new F